High-voltage converters on a thyristor-transistor generator. Scheme, description

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High-voltage converters on a thyristor-transistor generator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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In everyday life and in production, high-voltage converters are often used, for example, in ionizers, welding oscillators for non-contact ignition of the arc, as well as in automotive ignition systems, etc.

We offer high-voltage converters built on a thyristor-transistor generator, the load of which is automobile ignition coils of the B116 or B117 type.

The scheme is shown in Fig.1.

High-voltage converters on a thyristor-transistor generator

This device differs in that a two-stage amplifier is connected to the output of the master oscillator (the emitter of transistor VT2), the output transistor (VT4) of which feeds the primary winding of the automobile ignition coil.

Protective elements are introduced into the high-voltage converter circuit: a blocking diode VD4, a current-limiting resistor R12 and a protective zener diode VD3. They protect the control circuit and the master oscillator from reverse voltage pulses, and the VD6 diode serves to protect the output transistor VT4.

The operation of the device can be represented as a classic type of non-contact ignition devices, i.e. without a switching thyristor VS1, and as a multi-pulse high voltage source on a switching thyristor.

The device according to the first variant works as follows.

When the power is turned on, the blocking transistor VT1 is opened by a low voltage level at the base due to the resistor R1, and the converter is turned off. When a positive voltage is applied to the control input, for example, from the crankshaft angle sensor, the transistor VT1 closes and enables the converter to work. A positive bias at the base of VT2 opens the transistor, which, in turn, leads to the opening of the transistor VT3. This transistor, due to the positive voltage on the emitter, opens the power transistor switch VT4, which closes the lower output of the primary winding of the ignition coil to ground according to the circuit.

The process of increasing current in the coil and storing energy in its magnetic field begins.

After the end of the process at the moment of ignition, the breaker contacts open the power circuit or the control voltage applied to the VT1 base disappears. Transistor VT1 opens, blocks the operation of the converter and. thereby cutting off the current through the ignition coil winding. At this point, the magnetic field disappears and a voltage is induced in the coil windings.

The disadvantage of this method, especially at low engine speeds, is the increase in the energy accumulation time in the ignition coil, since the switching frequency of the breaker contacts decreases. The energy given off by the output transistor is unnecessarily spent on heating the coil and the transistor itself.

In this case, a single high-voltage pulse supplied to the candle may not provide reliable ignition.

Let's consider the second option. When a positive voltage is applied to the base of the transistor VT1, it closes. A positive voltage at the base of VT2 opens it, respectively, VT3 and VT4 open. At the same time, a positive voltage at the VT2 emitter through R7 and R4 opens the thyristor VS1. Opening, VS1 shunts the VT1 base to the body, and it closes, as a result of which VS1 closes, and again a positive bias appears on the VT1 base. Then the cycle repeats until the disappearance of the positive pulse based on VT1.

With an increase in engine speed, the ignition system switches from a multi-pulse mode to a single-pulse mode in cases of alignment of the switching frequency of the control input and the frequency of the converter master oscillator. The output voltage swing of the device is corrected by selecting a capacitor C5 and a resistor R11 or a capacitor filter and a zener diode are connected in parallel to the transistor VT4.

During testing, the performance of the circuit was checked with an ignition coil of the type. B117 autotransformer type without protective elements VD3, VD4, VD6, R12 and capacitor C3. The maximum breakdown distance for a spark in a candle reached 40 mm (15 mm is enough for an ignition system).

The diagram in Fig. 2 shows the control of the output stage of the converter using the optocoupler VU1, PC817 f. SHARP.

High-voltage converters on a thyristor-transistor generator

The optocoupler LED is connected to the collector circuit of the master oscillator transistor VT2, and the optocoupler phototransistor switches the VT3 transistor.

Devices according to the diagrams in Fig. 1 and 2 can also work with other loads, for example, regulate the speed of a DC motor.

Figure 3 shows a device for switching an accumulation lamp with a power of up to 100 W. The lamp blinking frequency is set by capacitors C1 and C3 and is selected by a trim resistor R5.

High-voltage converters on a thyristor-transistor generator

To smoothly adjust the incandescence of the lamp or the speed of the DC motor, it is necessary to reduce the capacitance of capacitors C1 and C3. In some cases, capacitors may not be installed. Then the maximum switching frequency of the lamp is obtained, imperceptible to the eye.

Authors: A. Alekseev, V. Alekseev, Perm

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